1. Field of the Invention
The present invention provides a solution to the problem of long-term wear between a caster and the mobile platform it is supporting. More particularly, the invention concerns a caster with a stem shaped for better engagement with the mobile platform it is supporting, thereby to reduce wear.
2. Description of the Related Art
Conventional casters are designed to support a mobile platform such as a shopping cart, an office chair, a scaffold, a truck, or a mechanic's creeper, above the ground. A caster is generally comprised of a wheel, a yoke, and a stem. The yoke is comprised of a generally horizontal base and two forks, the forks being generally parallel to each other and extending downwardly from the base. The ends of the forks support the wheel axle which in turn supports the caster wheel. The caster wheel is mounted so as to rotate freely about an axle, usually a nut and bolt axle or a riveted axle, which defines the horizontal axis of the wheel. Bearings are usually provided to decrease friction between the wheel hub and the axle and thereby improve the life of the wheel. Bearings may be selected from a wide variety of types including as roller, flanged, ball, delrin, and plain bore bearings.
Extending upwardly from the base is the stem, also referred to as a post, a load supporting member, a king pin or a pintle. The base is mounted on the stem in a manner which permits the yoke and wheel to turn about the vertical axis of the stud. The horizontal axis of the wheel is laterally offset from the vertical axis of the stud, and the wheel is thus free to rotate about the horizontal axis located behind the axis of the stud, and to pivot around the axis of the stud, when the mobile platform is in motion. The pivotability of the wheel and yoke with respect to the stem is usually achieved by supporting the yoke base between an upper set of ball bearings and, usually, also a lower set of ball bearings.
The caster is secured to the mobile platform by introducing the stem into a bore hole in the mobile platform so that the wheel of the caster extends down from the bottom of the platform. The stem may be provided with a high friction or self locking feature such that it remains in place once seated. Alternatively, the stem may be externally or internally threaded such that a correspondingly threaded metal fastener can be screwed onto or into the upper end of the stem to thereby securely fasten the caster to the mobile platform. For example, as shown in U.S. Pat. No. 5,527,051, the stem may be externally threaded, and a hollow internally threaded cap with a flanged top is threadably secured to the stem which extends upwardly through the bore hole from the bottom of the mobile platform.
Casters of this type are well known in the art and representative examples can be found in, e.g., U.S. Pat. No. 4,788,741 "Keyed Mounting Assembly for Lockable Swivel Caster"; U.S. Pat. No. 3,942,220 "Caster Sleeve"; U.S. Pat. No. 4,361,930 "Caster Socket Assembly"; U.S. Pat. No. 5,199,131 "Thermoplastic Caster Assembly"; U.S. Pat. No. 5,167,048 "Caster Wheel Retention Device"; U.S. Pat. No. 4,706,328 "Pivoted Caster Wheel and Yoke Brake Having a Detent"; U.S. Pat. No. 5,287,594 "Shopping Cart Swivel Yoke Assembly with Plastic Bearing Races"; U.S. Pat. No. 4,759,097 "Wheel Bracket Assembly Having Two PartWheel Bracket and a Method of Making the Wheel Bracket Assembly"; U.S. Pat. No. 5,527,051 "Mechanic's Creeper"; U.S. Pat. No. 5,012,550 "Caster With Brake"; U.S. Pat. No. 3,388,419 "Caster Locking Means"; U.S. Pat. No. 4,998,320 "Apparatus for Movably Supporting Beds and the Like"; U.S. Pat. No. 4,941,552 "Castor with Brake Mechanism"; and U.S. Pat. No. 5,331,717 "Automatic Locking Mechanism for Entraining Castor wheel Vehicles", these disclosures with respect to wheels, yokes, and swivel stems are incorporated herein by reference.
While a stem secured in a conventional manner may appear to be very stable when new, the stem fit does tend to wear out after extended normal use. This is because a small amount of play naturally exists between the stem and the bore hole, and after a heavily loaded mobile platform is pushed or pulled over rough ground over long periods of time, vibration wears the socket and loosens the metal fastener. Further, since the caster is designed to swivel, and since the bore hole and stem are generally cylindrical, this swiveling tends to transmit some torque to the kingpin, causing it to unscrew from the metal fastener over time. Once the stem starts to fit loosely in the borehole of the mobile platform, the fit begins to deteriorate rapidly, and repair of the condition is difficult.
One approach to addressing this problem involved drilling a hole laterally through the stem (and mobile platform) and inserting a pin such as a cotter pin through the hole to prevent rotation between the stem and mobile platform. However, such a pin is easy to break, difficult to remove when worn, and presents additional cost in drilling and assembly, besides being less attractive.
It is also known to lock a caster shaft against rotation by use of a lock-and-key form-fitting means as disclosed generally in patents relating to locking wheels, such as U.S. Pat. No. 4,788,741 to Hillborn. Hillborn teaches a caster including a mechanism for locking the caster in place such that the caster cannot pivot relative the vertical axis. This is accomplished by use of, on the one hand, a brake fork arm 22 which, when pressed, prevents the wheel from rotating or swiveling relative to the stem 18. The stem, on the other hand, is generally cylindrical, but has a longitudinal groove. The socket which receives the stem is provided with an inwardly projecting rib. The rib of the socket projects into the longitudinal groove of the stem to prevent rotation of the stem. When the brake fork is actuated, the caster wheel is prevented from rotation about the vertical axis and when the brake fork is deactivated the caster wheel is free to pivot relative to the vertical axis.
However, this caster is designed to be seated in a well in a secondary socket which, in turn, is securely received in the bottom of a hollow leg. The requirement for such a secondary socket represents additional expense in parts and assembly time, and also introduces additional play in the fit between the caster stem and mobile platform as is evident from FIG. 4. Further, such a stem is retained by seating a resilient split ring in a groove in the secondary socket. This makes the caster easy to remove, and unsuitable for heavy duty use such as for a shopping cart or mechanic's creeper. Further, the Hillborn caster stem assembly is not suitable for uses in which the caster must be fastened to a mobile platform with little or no projection above the mobile platform. For example, in a truck, shopping cart, or creeper, a mobile platform must be made as thin as possible. The caster must be securely attached to this thin mobile platform, yet may not project above the mobile platform at any point since any projection above the upper surface of the mobile platform will damage packages placed on the truck or shopping cart, or will hurt or inconvenience a mechanic laying on the creeper.
U.S. Pat. No. 3,388,419 entitled "Caster Locking Means also teaches a locking caster, wherein the stem of the swivel mechanism has a cubic outward appearance. However, no specific means are disclosed for securing the stem to the mobile platform. Further, the square stem shown in this patent can not fit into a conventional bore hole, since a square peg can not be made to fit into a round hole.
In summary, there appears to be no recognition in the art of the problem of wear and loosening of the fit between caster stems, mobile platform, and metal fasteners overtime. In particular, there is no teaching of a caster which can be mounted on a narrow frame with upper mounting hardware fitting flush against the upper surface of the mobile platform or frame, which caster design renders it is simple to install, economical to produce, and capable of installing into either conventional frames or frames having bore holes specifically adapted to receiving them.
There is thus a need for a caster which can be simply and economically produced, which is simple to seat and secure to a narrow mobile platform, and which remains securely seated over long periods of time even after prolonged exposure to vibration and stress. Such a caster should preferably be capable of being seated not only in a bore hole of special design, but also be capable of being seated in a conventional cylindrical bore hole. Thus, casters according to the present invention should be capable of replacing not only other special casters, but also be capable of replacing conventional casters.